The present invention relates to non-linear optical materials, and, in particular, to the growth of these materials, and, more particularly, to the hydrothermal growth of non-linear optical crystals.
Non-linear optical materials such as bismuth silicate may be used in optical memories and for optical signal processing. Bismuth silicate is especially desirable since it operates at higher speeds than other available non-linear optical materials.
Single crystals have conventionally been grown from a melt by using the floating zone technique, the Bridgman technique or the Czochralski technique. If a crystal is grown from a melt, the composition of the crystal is a function of the composition of the melt and of the growth conditions. Variations of the growth rate, for example, cause variations in the composition along the length of the crystal, but it is difficult to avoid all changes in growth rate. For many applications it is desirable that the composition of a single crystal should be substantially constant along the length of the crystal, both on account of the greater tendency of non-homogeneous crystals to crack during cutting or polishing operations and for the requirement of producing crystal elements having physical properties within narrow ranges of values.
Using the Czochralski growth technique a crystal is pulled from a melt of the same material. An oriented seed is dipped into the melt and slowly raised in such a way that the molten material freezes on the bottom of the seed due to the lower temperature as it is raised into the air above the molten material. The higher thermal conductivity of the solid also helps lower the temperature at the molten interface which aids in the freezing of the crystal. This is the most used crystal growth process and is particularly useful for crystals composed of a single element such as silicon. The quality of bismuth silicate crystals grown by this technique is marginal at best. The stoichiometry may not be satisfactory and certain impurities are introduced into the grown crystal which cause undesirable coloring of the crystal. A growth temperature related defect influences the "instrinsic" color and thus the non-linear optical properties. Internal strain in conventional crystals is evidenced under crossed polarizers. In addition, the reproducibility between runs is poor which means that each crystal must be separately evaluated for its optical properties prior to its application.
Until now, it has only been possible to grow bismuth silicate directly from the melt using the Czochralski technique or a variation of this technique called "top seeded solution growth" where a solvent is added to the molten solution which lowers the melting point of the melt making it possible to grow at a lower temperature. The reproducibility between runs is poor by either of these techniques which requires that each crystal be evaluated optically.